Bird flu too deep in human lungs to spread easily

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Two medical research teams have discovered why the bird flu is not spreading easily from person to person: The virus attaches itself too far down in the human respiratory tract to spread via coughs and sneezes, the usual way influenza spreads.

The findings were reported yesterday in the journal Nature by a group led by virologist Dr. Yoshihiro Kawaoka at the University of Wisconsin at Madison. Similar results by Dutch researchers also were published yesterday in the online journal ScienceXpress, Sciencexpress.org.

This helps explain why all people who have been stricken with the bird-flu virus, H5N1, became sick after directly handling infected poultry, rather than by having contact with infected people. The studies suggest that it will not be as easy as some had thought for H5N1 to mutate for efficient human-to-human transmission.

"Deep in the respiratory system, [cell] receptors for avian viruses, including avian H5N1 viruses, are present," Dr. Kawaoka said. "But these receptors are rare in the upper portion of the respiratory system. For the viruses to be transmitted efficiently, they have to multiply in the upper portion of the respiratory system so that they can be transmitted by coughing and sneezing."

Dr. Kawaoka, professor of pathobiological sciences at UW-Madison School of Veterinary Medicine, says his findings do not eliminate the threat of a bird-flu pandemic.

"Flu viruses constantly change," he said. "Certainly, multiple mutations need to be accumulated for the H5N1 virus to become a pandemic strain."

Avian flu has afflicted 184 persons in eight Asian countries and killed 103 of them.

Both the Kawaoka team and researchers at Erasmus Medical Center in Rotterdam, Netherlands, the authors of the report in ScienceXpress, used human tissue removed from various parts of the respiratory system to study where infection with H5N1 and human flu viruses occur in humans.

Investigators working with Dr. Kawaoka found that only cells deep within the human respiratory system -- usually on lung cells -- have the receptors or surface molecules necessary to allow the avian flu virus to enter a cell. If a virus cannot enter a cell, it cannot make particles that infect other cells, Dr. Kawaoka explained.

In contrast, human flu viruses tend to latch on to areas higher in the respiratory system, such as the nose.

"Our findings provide a rational explanation for why H5N1 viruses rarely infect and spread from human to human, although they can replicate in the lungs," the Wisconsin virologist said.

Asked about the joint discoveries, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, said this could be one factor in why H5N1 "has difficulty going from human to human."

"The virus' ability to evolve would probably involve a complex series" of events, not just one, he said.